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SAMPLING GEOPRESSURED FLUIDS: SOME CONTRIBUTIONS

FROM THE PROPERTIES OF THE H20-CH4 SYSTEM

Eduardo R . I g l e s i a s Ear th Sciences Div is ion

Lawrence Berkeley Laboratory Univers i ty of C a l i f o r n i a

Berkeley, C a l i f o r n i a 94720

I . INTRODUCTION

The geopressured formations of t he United S t a t e s Gulf Coast are being probed f o r methane recovery f e a s i b i l i t y . One of t he c r i t i c a l v a r i a b l e s involved i s t h e amount of methane a c t u a l l y d isso lved i n t h e pore b r i n e s . Sampling and subsequent a n a l y s i s o f t hese geopressured f l u i d s i s t h e r e f o r e important f o r t h e economic assessment of t h e resource . Thus, i n t e r e s t i n u s e of conventional downhole f l u i d samplers a n d , r e c e n t l y , i n develop- ment of samplers e s p e c i a l l y designed f o r geopressured environ- ments, has been s t imu la t ed .

The purpose of t h e s e t o o l s i s t o ob ta in f l u i d samples a t r e s e r v o i r cond i t ions and t o br ing them t o the su r face , p reserv ing t h e i r i n t e g r i t y , f o r subsequent chemical a n a l y s i s . The sampling process may be envis ioned as t h e fol lowing s impl i f i ed sequence. F i r s t t he sampler i s lowered along t h e wel lbore t o t h e r e s e r v o i r depth ( h e r e a f t e r t he bot tomhole) . A t t h e bottomhole t h e s a m p l e r i s f i l l e d with formation f l u i d . The v a l v e ( s ) a r e then closed and t h e sampler i s pul led back t o t h e su r face . There it i s housed i n t h e wellhead l u b r i c a t o r . A v a l v e i s o l a t i n g t h e l u b r i c a t o r from t h e wel lbore i s then c losed . The next s t age i s t o d ispose of t h e h o t , high pressure f l u i d contained i n t h e l u b r i c a t o r i n order t o reach t h e sampler. The sampler i s then recovered. F i n a l l y , t he f l u i d enclosed i n t h e sampler i s t r a n s f e r r e d t o s u i t a b l e con ta ine r s f o r subsequent chemical a n a l y s i s .

Pore f l u i d s i n geopressured formations a re subjec ted t o very high (up t o 1400 a t m d o 0 0 0 p s i ) p re s su res , and moderately high (up t o 20OoC) temperatures ( e . g . , Dorfman and F i s h e r , 1979); s i g n i f i c a n t amounts of methane, sodium ch lo r ide and lesser chemical spec ie s are d i s so lved i n t h e formation waters . During t h e sampling process descr ibed above, t h e temperatures of t h e f l u i d i n t h e s a m p l e r i n t h e wel lbore may depa r t s i g n i f i c a n t l y from t h e common bottomhole va lue ; fur thermore, t h e p re s su re of t h e f l u i d surrounding t h e sampler decreases with decreas ing depth. Correspondingly, va r ious e f f e c t s a s soc ia t ed with t h e thermodynamic p r o p e r t i e s of t h e f l u i d s would be induced. These e f f e c t s may inc lude ; gas exso lu t ion , both i n t h e sampler and i n t h e wel lbore ; v e r t i c a l composi t ional changes along

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t h e wel lbore due t o s l i ppage between t h e gas and t h e l i q u i d phase; temperature-induced pressure drops i n t h e f l u i d sample; s u b s t a n t i a l changes of t h e d i f f e r e n t i a l p ressure exe r t ed on t h e s a m p l e r w a l l s , which r e l a t e t o poss ib l e l eaks ; and formation i n t h e sampler of a s o l i d methane hydra te .

P red ic t ion of such e f f e c t s , and e s t ima tes of t h e i r magnitudes a r e use fu l f o r both u s e r s and des igners of downhole geopressured f l u i d samplers. For example, t h i s knowledge may h e l p i n t e r p r e t f i e l d r e s u l t s , be used i n assessment of sampling cond i t ions t o avoid those t h a t favor leakage of t h e sampler, and suggest s a f e r procedure f o r handl ing t h e sampler i n su r face ope ra t ions , as w i l l be shown i n t h i s paper.

This paper i s devoted t o p red ic t ing and q u a n t i t a t i v e l y e s t ima t ing geopressured f l u i d behavior dur ing sampling of r e s e r v o i r s i n t h e i r n a t u r a l , unperturbed cond i t ions . Both the f l u i d i n t h e sampler and t h e wel lbore f l u i d are considered. To t h a t end, I have developed a simple model ( an ' 'equation of s t a t e " ) t o es t imate thennophysical p r o p e r t i e s of geopressured f l u i d s . This model i s b r i e f l y descr ibed i n Sec t ion 11; f u l l d e t a i l s are given elsewhere ( I g l e s i a s , 1980) .

I n Sec t ion I11 t h e "equation o f s ta te" i s appl ied t o compute and d i s c u s s f l u i d p r o p e r t i e s a s soc ia t ed with the d i f f e r e n t s t ages of t he sampling process . Questions explored inc lude : t h e probable range of CH4 content of t he s a m p l e s ; p re s su re , phase t r a n s i t i o n s , f r a c t i o n of t o t a l volume corresponding t o each phase, and composition of each phase present i n t h e sample, over t h e expected range of temperatures; whether and under what condi t ions t h e f l u i d c o l l e c t e d a t wellhead i n a flowing w e l l provides a r e p r e s e n t a t i v e sample of t h e bottomhole f l u i d composition; t h e expected range of f l u i d p re s su res i n t h e l u b r i c a t o r ; and t h e expected range of d i f f e r e n t i a l s t r e s s e s on t h e s a m p l e r . Bottomhole t e m p e r a t u r e s and p res su res g e n e r a l l y inc rease with depth i n t h e geopressured formations of t h e Gulf Coast ( e . g . , Dorfman and F i s h e r , 1 9 7 9 ) . Thus, two w e l l dep ths , r ep resen t ing approximatley t h e t o p and t h e bottom of t h e geopressured zone, w e r e considered i n d e t a i l t o a s s e s s e f f e c t s a s soc ia t ed with depth.

F i n a l l y , r e s u l t s and recommendations a r e summarized i n Sec t ion I V .

11. A SIMPLE MODEL FOR GEOPRESSURED FLUIDS

With t h e except ion of a c o r r e c t i o n f a c t o r f o r methane s o l u b i l i t y i n N a C l s o l u t i o n s , I neglec ted t h e complicat ions posed by t h e presence of d i sso lved s o l i d s and considered a s y s t e m composed only of water and methane. This approach i s appropr i a t e f o r t h e e s t ima t ive purposes of t h e present work.

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In t h i s model, t h e thermophysical p r o p e r t i e s of t he water- methane mixture are formulated i n terns of f i v e main v a r i a b l e s ; namely, p ressure P , abso lu t e temperature T , molar volume v , mole f r a c t i o n o f methane i n the - sys t em A , and volumetr ic gas s a t u r a t i o n S . The c o n t r i b u t i o n s of methane t o t h e l iqu id- and gas-phase molar volumes are es t imated from a c o r r e l a t i o n ( B r e l v i and O'Connell, 19721, and from t h e i d e a l gas l a w , r e s p e c t i v e l y . The corresponding q u a n t i t i e s f o r l i q u i d water and steam a r e es t imated from t h e IFC Formul a t ion".

Methane s o l u b i l i t y i s computed from an empir ica l c o r r e l a t i o n (Haas, 1978) , which assumes t h a t steam e x i s t s i n t h e gas phase a t i t s s a t u r a t e d pressure and d e f i n e s t h e mrathane p a r t i a l p ressure as t h e d i f f e r e n c e between P and t h e s a t u r a t i o n pressure of pure steam.

111. RESULTS AND DISCUSSION

i> Bottomhole Compositions

The geopressured b r i n e s of t h e Gulf Coast are be l ieved t o be s a t u r a t e d with d i s so lved methane. Methane s o l u b i l i t i e s i n water and i n N a C l s o l u t i o n s depend on t e m p e r a t u r e and pressure . Thus, given t h e ranges of p re s su res and temperatures found i n t h e geopressured formations, t h e probable range of methane conten t i n bottomhole f l u i d samples can be es t imated from known s o l u b i l i t i e s , as fol lows.

Bubble-point curves f o r t h e water-methane system were computed from Haas' c o r r e l a t i o n . These r e s u l t s a r e shown i n Figure 1. The shaded area r e p r e s e n t s , roughly, t h e P , T ranges spanned by t h e f l u i d s of t h e Gulf Coast ( e . g . , Dorfman and F i s h e r , 1979 and r e fe rences t h e r e i n ) . Assuming t h a t i n t h e unperturbed r e s e r v o i r t h e pore water i s s a t u r a t e d wi th methane, CH4-H20 r a t i o s can be es t imated from Figure 1 f o r g iven p res su res and temperatures . Mult iplying rhese r e s u l t s by t h e f a c t o r fNaCL, which has been p l o t t e d i n F igure 2 as a func t ion of t h e N a C l con ten t , c o r r e c t s t h e s e s o l u b i l i t i e s f o r s a l i n i t y (Haas, 1978).

From Figure 1, t h e maximum methane s o l u b i l i t y i n t h e shaded area ( f o r zero NaCl.) i s about 14,400 ppm a t 1400 a t m ( t h i s p re s su re corresponds approximately t o t h e bottom of t h e geopressured zone). This s o l u b i l i t y may dec rease t o about 5000 ppm, corresponding t o fNaC1 3 0.35 f o r 250,000 ppm of N a C l (F igure 2 ) . Taking 700 a t m (corresponding t o about 3000 m , t h e t o p of t h e geopressured zone, i f t h e l i t h o s t a t i c p re s su re g rad ien t i s adopted) , t h e minimum methane s o l u t i b i l i t y is about 4000 ppm f o r zero N a C l

" In t e rna t iona l Formulation Committee, 1967, "A Formulation o f t h e Thermodynamic P r o p e r t i e s of Ordinary Water Substance," IFC S e c r e t a r i a t , Verein Deutscher Ingenieure , Dusse ldor f , Prinz-Georg-Strasse 77/79.

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(F igure 1) which conver t s t o about 1400 ppm f o r 250,000 ppm of N a C 1 .

Two cases were considered i n d e t a i l : a deep w e l l ( Z B = 6000 m) r ep resen t ing an approximate upper l i m i t t o t h e pressures expected, and a "shallow" ( Z B = 3600 m) w e l l sunk t o near t o the top o f t h e geopressured formations. The corresponding bottomhole p re s su res were es t imated from t h e l i t h o s t a t i c g rad ien t . Bottomhole temperatures were then picked from Figure 1, and methane s o l u b i l i t i e s i n pure water computed from inve r s ion of P(X,T) as def ined by Haas. These r e s u l t s are presented i n Table 1.

i i ) Wellhead

It i s appropr i a t e t o cons ider he re t h e quest ion of whether and under what cond i t ions does t h e fllvid co l l ec t ed a t wellhead from a flowing w e l l provide a r e p r e s e n t a t i v e sample of t h e bottomhole f l u i d composition.

Since u n t i l t he present t ime no geopressured r e s e r v o i r has been under s i g n i f i c a n t product ion, t h e scope of t h i s paper i s l imi t ed t o unperturbed r e s e r v o i r s . In these condi t ions t h e pore f l u i d s are be l ieved t o c o n s i s t of a s i n g l e , l i q u i d phase s a t u r a t e d with methane. Wellbores pene t r a t ing geopressured r e s e r v o i r s are f i l l e d with formation f l u i d due t o t h e high pore pressures . Along t h e wel lbore pressure decreases wi:h decreas ing depth causing gas (mostly methane but a l s o some steam) exso lu t ion . Buoyancy then t ends t o sepa ra t e t h e gas bubbles from t h e parent l i q u i d . This e f f e c t may cause t h e f l u i d composition a t wellhead t o d i f f e r s i g n i f i c a n t l y from t h e bottomhole composition.

To estimate t h e wellhead o v e r a l l composition, s l i ppage between t h e gas and t h e l i q u i d phase must be considered. The t y p i c a l s l i ppage l eng th , A Z , a t wellhead car, be der ived from Z B and t h e r a t i o of t h e bubble- l iquid r e l a t i v e v e l o c i t y , v r t o t h e flow v e l o c i t y v f ,

= ZB (Vr/Vf)

The extremely high bottomhole p re s su res imply nea r sonic flow v e l o c i t i e s when f r i c t i o n l o s s e s a r e considered (C. Miller, p r i v a t e communication). Sound v e l o c i t i e s may range from approximately 1000 m t o perhaps 100 m s-l , depending on gas s a t u r a t i o n ; a t y p i c a l va lue for vr i s 0.5 m ( e .g . , Haberman and Morton, 1953). Thus A 2 may range up t o a few hundred meters f o r geopressured w e l l s . One can t h e r e f o r e neg lec t s l ippage and t ake Xw = AB i n t h e s e f a s t f lowing wel l s . small bubble regime has been t a c i t l y assumed f o r t h e wel lbore flow. Due t o the r e l a t i v e l y small temperature g rad ien t s and high p res su res involved, t h i s i s a very reasonable assumption; t h i s assumption i s supported by t h e small gas s a t u r a t i o n s found l a t e r i n t h i s paper.

In t h i s argument,

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To i n v e s t i g a t e the d i f f e r e n t i a l s t resses exer ted on t h e sampler at su r f ace l e v e l , es t imates of wellhead pressures and temperatures are needed. For s i m p l i c i t y I considered s t eady- s t a t e , f a s t f lowing w e l l s f o r t he two w e l l depth cases descr ibed above.

Neglect ing drawdown and f r i c t i o n e f f e c t s a s soc ia t ed with f i n i t e flow v e l o c i t y , t h e wellhead pressure PW w a s approximated as t h e bottomhole p re s su re minus the h y d r o s t a t i c head. The assumed s t eady- s t a t e flow cond i t ions imply wellhead temperatures TW not f a r from Tg. Thus, f o r convenience I assumed a l i n e a r temperature p r o f i l e f o r t h e wel lbore , with TW ranging from TB t o (Tg - 5 O O C ) . These temperatures r e s u l t i n small gas s a t u r a t i o n s i n t h e we l l , which a r e n e g l i g i b l e i n terms of mass. Neglect ing the con t r ibu t ions of t h e gas phase and of t h e small amounts of d i sso lved methane t o t h e t o t a l d e n s i t y , I approximated P 1 Z P k ( P , T ) . computed f o r t h e two wel l depths considered A r e shown i n Table 1. Note t h e small d i f f e r e n c e s i n Pw a r i s i n g from t h e temperature dependence of t h e dens i ty .

Resul t s so

From Pw, Tw, and lw, which completely spec i fy the thermo- dynamic s t a t e of t h e system, o t h e r wellhead v a r i a b l e s of i n t e r e s t such as S , x, y and r were computed using the "equation of s t a t e . " The corresponding results are summarized in T a b l e 1.

In add i t ion t o y i e ld ing t h e wellhead cond i t ions sought , t hese r e s u l t s provide seniquant i t a t i v e informat ion o f i n t e r e s t f o r planning a c t u a l product ion. This information can be summarized as fol lows: i f a t bottomhole cond i t ions t h e b r i n e i s sa tu ra t ed with methane, gas w i l l f i r s t evolve wi th in t h e wellbore e a r l y i n t h e product ion h i s t o r y , but t h e concomitant drawdown w i l l even tua l ly r e s u l t i n phase sepa ra t ion wi th in t h e formation. The volume f r a c t i o n corresponding t o t h e gas phase anywhere along t h e wel lbore i s smal l , most l i k e l y less than about 4 % . Therefore , t h e flow i s expected t o be i n t h e s m a l l bubble regime. A s u b s t a n t i a l f r a c t i o n of t h e t o t a l methane remains i n s o l u t i o n a t wellhead: about 50% i f thermal l o s s e s along t h e wel lbore are s i g n i f i c a n t , and s u b s t a n t i a l l y more other- w i s e f o r t h e cases considered (Table 1 ) . However, t h e computed va lues of r a r e upper l i m i t s because t h e a c t u a l wellhead p res su res w i l l be smaller than shown i n Table 1 due t o neglec ted f r i c t i o n lo s ses and drawdown, and consequent ly thc-re w i l l be g r e a t e r methane exso lu t ion . Note t h a t t hese r e s u l t s apply t o t h e e a r l y s t ages of product ion; i . e . , be fo re a gas phase develops i n t h e r e s e r v o i r . Af t e r s epa ra t ion of t h e f l u i d i n t h e r e s e r v o i r s l i ppage may become non-negl igible .

w

i i i ) Sampler

This subsec t ion focuses on thermodynamic changes ( p r e s s u r e s , phase t r a n s i t i o n s , e t c . ) t ak ing p lace i n t h e f l u i d sample over t h e expected range o f temperatures .

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Neglecting l eaks and thermal expansion e f f e c t s , t h e s a m p l e r can be regarded as a c losed , i sochor i c s y s t e m . On t h i s b a s i s , f l u i d v a r i a b l e s of i n t e r e s t were computed as func t ions of temperature as the f l u i d i s cooled from TB t o 25OC. The necessary i n i t i a l cond i t ions ( P B , TB, A B ) were taken from Table 1.

Gaseous methane and steam evolve i n the process . The corre- sponding gas s a t u r a t i o n va lues range from zero a t bottomhole temperatures t o less than 10% a t 2 5 O C , and increase ( i . e . , c o r r e l a t e , v i a t h e m u l t i p l e c o r r e l a t i o n l i nk ing depths , tempera- t u r e s and p res su res of geopressured f l u i d s ) with wel l depth (F igure 3 ) . Methane i s computed (F igure 4 ) t o c o n s t i t u t e i n excess of 98 mole % of the gas phase over the range of temperatures considered; however, our model probably underest imates the gas phase steam mole f r a c t i o n . The f r a c t i o n of t o t a l methane remaining i n s o l u t i o n c o r r e l a t e s nega t ive ly with wel l depth; r decreases a s cool ing proceeds, u n t i l a minimum, whose p o s i t i o n i s i n s e n s i t i v e t o wel l depth , i s reached near T = 50°C (Figure 4 ) . The minimum value of r ranges upward of 20% ind ica t ing t h a t cons iderable methane exso lu t ion w i l l t ake place upon depres su r i za t ion f o r s a m p l e t r a n s f e r , even a t near ambient temperatures .

A s expected, t h e sampler 's f l u i d pressure c o r r e l a t e s with wel l depth. Cooling e f f e c t i v e l y decreases the s a m p l e r ' s f l u i d p re s su res from bottomhole va lues of up t o about 1400 atm a t 2OO0C t o a maximum value of about 300 atm a t 2 5 O C , where c a l c u l a t i o n s were terminated (F igure 3 ) . These r e s u l t s imply t h a t formation of a methane hydra t e , t h a t t akes p lace a t p ressures i n excess of 4 6 3 atm a t 25OC (Kobayashi and Katz, 1 9 4 9 ) , w i l l not c o n s t i t u t e a problem i f t h e s a m p l e r i s cooled t o t h a t temperature . However, t h e p o s s i b i l i t y of methane hydra te for- mation i n and nea r t r a n s f e r va lves e x i s t s because of poss ib l e l o c a l overcool ing due t o dep res su r i za t ion , i f t r a n s f e r i s attempted a t near ambient temperatures . This problem should be e a s i l y c o n t r o l l a b l e by use of l o c a l hea t ing ( e . g . , e l e c t r i c wires) o f t h e a f f e c t e d zone.

i v > D i f f e r e n t i a l P res su re

In t h i s s e c t i o n we cons ider t h e d i f f e r e n t i a l p ressure on the s a m p l e r , which i s def ined as

P = P (sampler f l u i d ) - P (surrounding f l u i d ) .

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A s shown above, t h e i n t e r n a l p re s su re of t h e sampler i s c o n t r o l l e d by t h e temperature, sample volume and composition being cons t an t . On t h e o t h e r hand, t h e wellbore f l u i d p re s su re i s mainly c o n t r o l l e d by t h e h y d r o s t a t i c head. The t y p i c a l t i m e taken t o b r ing t h e sampler back t o t h e su r face ( a few hours ) cons iderably exceeds t h e thermal e q u i l i b r a t i o n t i m e between t h e f l u i d contained i n t h e m e t a l l i c sampler and t h e surrounding f l u i d . Thus, i n t h e journey t o t h e su r face t h e f l u i d sample temperature follows t h e temperature p r o f i l e of t h e wel l . Therefore , t h e i n t e r n a l and e x t e r n a l p re s su res along t h e wellbore must be compared a t t h e wellbore temperature.

Assuming, f o r t h e sake of t h e argument, approximately l i n e a r p r o f i l e s f o r P and T, t h e wellbore p re s su re a t a given depth i s p ropor t iona l t o t h e corresponding temperature. This l i n e a r r e l a t i o n s h i p , i f superimposed on t h e P-T diagrams of F igures 3 or 7 , would appear as s t r a i g h t l i n e s (one f o r each well depth) running between (Pg, TB) and (Pw, Tw), t h e p o i n t s corresponding t o bottomhole and wellhead cond i t ions r e s p e c t i v e l y . For given bottomhole cond i t ions t h e s e s t r a i g h t l i n e s would p ivot around (PB, TB) i f P, or Tw are v a r i e d . head. t h e s t r a i g h t l i n e s r ep resen t ing t h e wellbore f l u i d p re s su re i n t h e P-T diagram. The s lope decreases wi th decreas ing va lues of T,. A t a given temperature t h e d i f f e r e n c e between t h e curve r ep resen t ing t h e f l u i d sample p res su re and t h e s t r a i g h t l i n e r ep resen t ing t h e wellbore f l u i d p re s su re f o r each well depth i s AP, t h e d i f f e r e n t i a l p ressure . For T, s u f f i c i e n t l y l a r g e , t h e s t r a i g h t l i n e l i e s below t h e f l u i d sample p re s su re , and AP i s p o s i t i v e . But decreas ing T, causes the s t r a i g h t l i n e t o p ivot around (PB, TB) towards h igh pressures, and e v e n t u a l l y AP becomes negat ive i n t h e wellbore. I n a c t u a l w e l l s t h e r e l a t i o n s h i p between wellbore f l u i d and temperature g e n e r a l l y shows some cu rva tu re . But t h e argument made above s t i l l a p p l i e s q u a l i t a t i v e l y . Thus, i n "hot" ("cold") w e l l s , AP t ends t o be p o s i t i v e (nega t ive ) .

As d i scussed , P, i s determined mainly by t h e h y d r o s t a t i c Therefore T, i s t h e main v a r i a b l e c o n t r o l l i n g t h e s lope of

A t wellhead, t h e sampler i s enclosed i n t h e l u b r i c a t o r . The f l u i d i n t h e l u b r i c a t o r i s i s o l a t e d from t h e wellbore f l u i d by means of va lves . In t h i s cond i t ion t h e f l u i d i n t h e l u b r i c a t o r is a t cons tan t volume and composition, n e g l e c t i n g l e a k s and thermal expansion. Thus, t h e l u b r i c a t o r f l u i d p re s su re i s c o n t r o l l e d by t h e temperature, given t h e i n i t i a 1 , v a l u e s of t h e composition A,, pressure P, and temperature Tw.

Using A,, P,, and T, from Table 1 as i n i t i a l cond i t ions , I computed temperature dependent l u b r i c a t o r f l u i d q u a n t i t i e s from t h e "equation of s t a t e " , a t cons tan t molar volume v and composition A , f o r both well depths cons idered . Two curves r e s u l t e d f o r each q u a n t i t y (F igu res 5 through 7 ) because f o r each w e l l , two d i f f e r e n t wellhead cond i t ions ( i . e . , A,, P,, T,) were considered. R e s u l t s c l o s e l y resemble those obtained f o r t he sampler.

-91 -

I n Figure 7 t h e f l u i d sample p re s su res a r e compared with t h e p re s su res of t h e surrounding f l u i d i n t h e l u b r i c a t o r a t t h e common equ i l ib r ium tempera tures . This F igure i n d i c a t e s t h a t i n t h e l u b r i c a t o r AP may be p o s i t i v e or negat ive . This comparison i s v a l i d f o r cases i n which the wellhead parameters dur ing sampling a r e comparable t o t h e wellhead parameters of f a s t flowing w e l l s , as def ined above. Negative va lues of AP are favored when (TB - Tw) t e n s of degrees C .

-several

These r e s u l t s a r e u s e f u l t o a s s e s s t h e performance of c e r t a i n samplers i n which t h e va lves are kept c losed by t h e combined p res su res of a sp r ing and of t h e i n t e r n a l ( o r e x t e r n a l ) f l u i d . Since t h e p re s su re exe r t ed by t h e s p r i n g s i s n e g l i g i b l e with r e spec t t o t h e f l u i d p re s su res involved, t h e s e samplers w i l l l e ak when AP i s nega t ive ( p o s i t i v e ) .

v ) Sampler Recovery

High (up t o about 840 atm) i n t e r n a l pressures are expected f o r t h e l u b r i c a t o r a t s t eady s ta te wellhead temperatures. Even h igher i n t e r n a l p re s su res , up t o e s s e n t i a l l y PB, are a l s o ind ica t ed f o r t h e sampler. These high p res su res a r e accompanied by high tempera- t u r e s i n a s a l i n e ambient which may inc lude s u l f u r (and o t h e r ) compounds, r e s u l t i n g i n e s p e c i a l l y favorable cond i t ions f o r micro- c rack development t h a t may r e s u l t i n c a t a s t r o p h i c m a t e r i a l f a i l u r e s . Rot, high p res su re l eaks through j o i n t s and va lves c o n s t i t u t e another unpleasant p o s s i b i l i t y . These circumstances bear not only on t h e m a t e r i a l a s p e c t s of sampling geopressued f l u i d s , but a l s o on t h e r i s k l e v e l faced by t h e crew i n charge of sampler recovery.

A simple procedure, which would s i g n i f i c a n t l y l e s s e n t h e m a t e r i a l and personal r i s k s concomitant with sampler recovery, i s suggested by t h e r e s u l t s of t h i s s e c t i o n . 'Itiis procedure c o n s i s t s of two s t e p s . F i r s t , t h e l u b r i c a t o r i s i s o l a t e d from t h e f l u i d flow by c l o s i n g appropr i a t e v a l v e s , t o minimize thermal con tac t . Then the l u b r i c a t o r i s e x t e r n a l l y cooled down t o near ambient temperatures. The second s t e p would s u b s t a n t i a l l y decrease t h e i n t e r n a l pressures of both t h e l u b r i c a t o r and t h e sampler (F igure 7). Thermal shocks on the sampler would be minimized t h i s way. This procedure has the added advantage of minimizing t h e d i f f e r e n t i a l p ressure f e l t by t h e sampler wal ls . For example, i n t h e extreme cond i t ions r.orresponding t o t h e deep w e l l case AP may reach (F igure 7 ) a maximum va lue of 554 a t m , as compared t o 1400 a t m i f no cool ing were performed; t h e va lue of AP would be reduced t o e s s e n t i a l l y t h e i n t e r n a l sampler p re s su re of 310 atm, a t recovery, i f t h e l u b r i c a t o r were brought t o 25% befo re p re s su re r e l e a s e .

A s imple model f o r t h e "equation of s t a t e " of t h e H20 - CH4 system has been used t o p r e d i c t f l u i d behavior during sampling ope ra t ions of unexploited geopressured r e s e r v o i r s of t h e United S t a t e s Gulf Coast. The main r e su l t s are as follows.

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The methane conten t of t h e f l u i d samples may vary widely, from about 1400 ppm t o about 14400 pm. The former f i g u r e corresponds t o hypo the t i ca l , h igh ly s a l i n e (250,000 ppm NaC1) samples from near t h e t o p of t h e geopressured zone; t h e l a t e r f i g u r e corresponds t o hypo the t i ca l , ve ry low s a l i n i t y samples from near t h e bottom of t h e geopressured zone. Sample methane conten t tends t o inc rease r ap id ly with inc reas ing bottomhole temperatures and p res su res , and t o decrease with inc reas ing s a l i n i t y .

When a geopressured w e l l i s i n i t i a l l y t e s t e d , t h e o v e r a l l ( l i q u i d and gas phases included) concent ra t ion of methane a t wellhead d i f f e r s n e g l i g i b l y from t h a t of t h e r e s e r v o i r f l u i d , i f l a r g e f lowra tes ( v e l o c i t i e s 2 100 m s - l ) occur . an approximate check on r e s u l t s obtained with samplers.

This provides

A t su r f ace temperatures (assumed t o range approximately from 25OC t o nea r ly 2OO0C), and before dep res su r i za t ion , t h e f l u i d samples c o n s i s t of a two-phase mixture ( l i q u i d and g a s ) , but gas s a t u r a t i o n s are s m a l l (5 10% by volume) and s i g n i f i c a n t f r a c t i o n s (2 20%) o f CH4 remain i n s o l u t i o n , i n d i c a t i n g cons iderable methane exso lu t ion upon depres su r i za t ion f o r sample t r a n s f e r . This informa- t i o n i s u se fu l i n planning procedures and hardware f o r f l u i d sample trans f e r .

Cooling r ap id ly decreases sample p re s su res , e .g . f rom bottomhole va lues of up t o about 1400 a t m a t 2OO0C t o a maximum value o f about 300 a t m a t 25OC. Therefore , formation of a methane hydra t e , t h a t t akes p lace a t p re s su res i n excess o f 463 a t m at 2 5 O C , i s not normally expected i n t h e sampler before t r a n s f e r . However, l o c a l overcool ing due t o d e p r e s s u r i z a t i o n concomitant with sample t r a n s f e r might cause methane hydra te formation i n and near t r a n s f e r va lves .

The d i f f e r e n t i a l p re s su res exer ted on the sampler may be p o s i t i v e o r nega t ive , depending on t h e wel lbore temperature p r o f i l e . Subs tan t i a l temperature g r a d i e n t s along t h e wel lbore (TB - Tw degrees C) favor s i t u a t i o n s where t h e e x t e r n a l f l u i d p re s su res exceed t h e i n t e r n a l pressure . This i n d i c a t e s t h a t samplers r e l y i n g on a combination of sp r ing and i n t e r n a l ( e x t e r n a l ) f l u i d pressure t o keep t h e va lve ( s ) c losed w i l l l e ak when used i n "cold", non-preheated (Ithot", preheated) we l l s .

s eve ra l t e n s of

F i n a l l y , a s i m p l e procedure t o reduce personal and m a t e r i a l r i s k s a s soc ia t ed with sampler recovery i s suggested. It c o n s i s t s of e x t e r n a l l y cool ing t h e c losed l u b r i c a t o r conta in ing t h e sampler t o near ambient temperatures . This would s u b s t a n t i a l l y decrease the i n t e r n a l pressures of both t h e l u b r i c a t o r and t h e sampler, and a l s o t h e d i f f e r e n t i a l p ressure exe r t ed on t h e sampler.

- . . . . . . . . . . . . . . . . . . . .. . . . . . . .

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ACKNOWLEDGMENTS

Th i s work w a s supported by t h e A s s i s t a n t Sec re t a ry €or Resource App l i ca t ions , Of f i ce of I n d u s t r i a l & U t i l i t y Appl ica t ions & Opera t ions , Div is ion of Geothermal Energy of t h e U.S. Department of Efiergy under Cont rac t No. W-7405-ENG-48. The au thor wishes t o acknowledge u s e f u l d i s c u s s i o n s wi th D r . C . M i l l e r of LBL.

REFERENCES

B r e l v i , S.W., and O'Connell, J.P., 1972, AIChE J., 18, 1239, "Corresponding S t a t e s C o r r e l a t i o n s f o r Liquid Compress ib i l i t y and P a r t i a l Molal Volumes of Gases a t I n f i n i t e Di lu t ion in Liquids."

Culberson , O.L., and McKetta, J. J., 1951, Petroleum Transac t ions AIME, 192, 223, "Phase E q u i l i b r i a in Hydrocarbon-Water Systems I11 - The S o l u b i l i t y of Methane in Water a t Pressures t o 10,000 ps ia . "

Dorfman, M.H., and F i s h e r , W.L., e d i t o r s , 1979. Proceedings of t h e Fourth United S t a t e s Gulf Coast Geopressured-Geothermal Werqy Conference: Research and Deve lopen t . Center f o r h e r q y S t u d i e s , t h e Un ive r s i ty of Texas a t Austin.

Haas, J . L . , 1978, USGS Open F i l e Report No. 78-1004 , "An Empirical Equation with Tables of Smoothed S o l u b i l i t e s of Methane in Water and Aqueous Sodium Chloride So lu t ions up t o 25 Weight Pe rcen t , 36OoC and 138 MPa."

Haberman, W.L., and Morton, R.K., 1953, David W. Taylor Model Basin Report 802.

I g l e s i a s , E.R., 1980. Samplhu Geopressured F lu ids : from Bottomhole t o Wellhead and Beyond. LBL-11310 Report.

Kobayas i , R. , and K a t z , D.L. , 1949 (March) , Trans. A I M E , T.P. 2579.

NOMENCLATURE Variables

fNaC-: Correction f a c t o r for CH4 s o l u b i l i t y in NaCl s o l u t i o n s n : mole number P: pressure

I= n / ( n L + n 2 ) :

S: volume f r a c t i o n corresponding t o t h e gas phase T :

L G 2 2

temper a t u r e

f r a c t i o n of t o t a l methane i n t he l i q u i d phase

W

W

m

m

0

0

0

0

03

00

0

0

0

0

P

P

wl

cn

0

0

P

P

0

cn

0

0

0

0

4

4

0

0

F.

c-

" m

m

" h,

N

Q

W

4

c-

c-

UI

m

F.

0

N

P

c-

VI

0

P

0

0

w

cn

4

P

W

W

W

03

4

W

03

03

cn

4

N

N

0

W

-94-

m

0

0

0

P

c-

0

0

N

0

0

ro

0

0

P

ln

cn

cn

F

c-

0

N

W - 0

3

c-

m

P

m

c-

i-

P

W

W

03

P

\D

4

m

0

w

0

R

R 0 5- 0 f

-95-

NOMENCLATURE Var iab les (cont inued)

v : molar volume x: CH4 mole f r a c t i o n in t h e l i q u i d phase y : CH4 mole f r a c t i o n i n t h e gas phase z: w e l l depth

A: = (n2L + n2G)/[(n2L + n2G) + (nlL + n lG>I : mole f r a c t i o n of methane i n t h e system

SuDer- and sub-indexes

B: bottomhole cond i t ions G : gas phase L: l i q u i d phase W: wellhead cond i t ions

1: H 2 0 2 : CHb

-96-

-

- -

-

-

- -

-

7

TEMPERATURE ('f)

IOG

991

99.1

991

B9.1

99.1

98.1

1200

- BGC -

w D m rn W

0

a

a

400

0

-

I I I I I I J 5c I5C 25C

TEMPERATURE ('C)

Fig. 1. Solubi l i ty curve. for the Rz- sywtm. ( p p ) r t h m e concentration. are indicated. Ihe ohadd area represento. a p p r o x b t e l y . the range of pressure ad tarpsrsture covered by the fo ru t i cm fluid. of the Gulf mast i n the United States.

Moot - DEEP WELL uDOt --- SHALLOW WELL

1000 I + 4 - = l o o = m m W

600

400

ZOO

0

/

I I I 1 I I 50 100 150 200

TEMPERATURE (TI

Fig. 2. sa l in i ty e o m c t i o n f x t o r . me aolubilitiew of CII,, in I(S1 brine. may be estimated by the product of fwd1 times the oolubi l i ty of methane in pure water at the s m prewwure md tmmperature.

- DEEP WELL --- SHALLOW WELL

I I

I I I I

I I I 1 I I I ] 50 IO0 I50 2QO

TEMPERATURE ('C)

Fig. 4. Rution of t o t a l mthwne in solution ad gas phame r t h m e wle fraction rsspcawes t o a r p l e r cooling for tw r l l dopthw.

Fig. 3 Rewsure wnd (volImatric) 9.8 waturacion reapcase0 of the ompler'o f luid t o cooling for tw -11 depth#: 6000 P (-1 and 3600 P (-).

-97 -

I ' I I I I I I I

- DEEP WELL --- SHALLOW WELL - DEEP WELL --- SHALLOW WELL

.

I I I I 01 ' 50 I00 I5 I 206

TEMPERATURE ('C)

Pig. 5. Cas sa tu ra t ims vs. t ape racu re fo r the (closed) Lubricator. u s l w d v a l w s f o r the s t e d p s t a t e wel lhad r ape ra tu re . u e shoun fo r aach n l l depth.

hn curves, corresponding to c w different

DEEP WELL I

--- SHALLOW FELL I

- /

I f

f / /

I . I I 1 - 50 IO 0 15C xc

TEMPERATURE ('C) Fig. 6. Rcc im of t o t a l r t h m e in solution vs. t ape racu re

for the (closed) lubricator. to tm different u s m d values for the steady-mtate n1lh.d t s p r a t u r e . u s sham for each w e l l depth.

hn curves. corresponding

I I 1 I I

; - DEEP WELL

I I I I I 1 50 100 150 202

TEMPERATURE ('C) Fig. 7. Fluid pressure vs. t a p e r a t u r e fo r the (clomd)

lubricator. u s d r a l w s for the steady-state a l l h e d tRaperature. are shorp tor a u h r l l dapth. For c m p u i m n , the F-T CIPI.. comaapoading t o the srp lar f luid for each r l l &pth are also s h a a .

Ica curves. corraspondily to CUO different